The present invention is directed to a method of generating a high voltage in an electrostatic powder coating gun and to an electrostatic powder coating gun.
Known powder coating devices generally have a high-voltage generator comprising a transformer and a high-voltage cascade arrangement at the transformer output side. The output voltage from the transformer is supplied to the low-voltage input terminal of the high-voltage cascade arrangement, and the output voltage from the high-voltage cascade arrangement is supplied to a high-voltage coating electrode. In the operation of the coating device, the primary coil of the transformer is fed from an a.c. power supply operated at a working frequency. To this end a power supply unit is normally provided with such known coating guns for outputting an a.c. supply voltage at a working frequency matched with the components of the high-voltage generator, in which the high-voltage transformer constitutes the frequency-determining portion of this "free-running oscillator", as it is called. The operation of the high-voltage generator depends on the individual components thereof such as the transformer, high-voltage cascade arrangement etc., as well as inductive and capacitive influences which may, for instance, be caused by length, cross-section and type of cable or by external disturbances.
When the coating device is activated, the above-described prior art oscillators must build up to the working frequency and are required during operation to maintain this working frequency to the greatest extent possible. The working frequency of the high-voltage generator is predominant in determining the primary coil current of the transformer. If possible, the high-voltage generator should operate at a frequency where the primary coil current is minimum so that the efficiency of the high-voltage generator is maximum. This minimum of primary coil current is significant, for example, with the cell coils frequently used in such high-voltage generators. FIG. 1 is an explanatory diagram showing the primary coil current I as dependent on the frequency F of the supply voltage, where three working points f.sub.1, f.sub.2 and f.sub.3 are illustrated. The various working points where the primary coil current minimum I.sub.min exists substantially depend on system design. The primary coil current will rise when the frequency of the supply voltage in a high-voltage generator deviates from the working point either in positive or negative direction due to system load or external interferences.
Various difficulties usually occur with the known systems. The high-voltage generator may be designed for a predetermined constant load with a stationary power unit, fixed cable lengths and a given transformer and high-voltage cascade arrangement, i.e. it may be designed for a working point at which the working frequency, the primary coil current, and the high voltage remain constant. But if the attenuation of the thus formed oscillator circuit varies due to different load currents, such a system is unable to correspondingly adapt to the change of conditions. This leads to a change in current input to the high-voltage generator and consequently the desired output voltage cannot be maintained, so that the efficiency is significantly deteriorated. Very high loads may result in a temporary complete failure of the high-voltage output. Also, the tolerances of the a.c. and d.c. amplification factors of oscillator transistors and feeder cables will act on the characteristics of the entire oscillating circuit. Moreover, it has been found in practical use that with greater cable lengths such oscillators exhibit substantial power loss or become unable to oscillate.